Odd pupils could be key to camouflage trick of colorblind octopi and squid

Despite their ability to change color to perfectly match their surroundings, cephalopods like octopi and cuttlefish are actually colorblind, having only one photoreceptor, unlike the three we humans possess. The way in which they can pull off their camouflage trick then, has been a source of speculation for scientists for some time. Now, father and son scientists from two universities believes they've cracked the mystery – and it all has to do with some strange pupils.

In the human vision system – and that of many other mammals as well – the eyes have relatively small pupils that focus light to a precise point on the cones contained in our retinas at the back of our eyes. This produces a sharp image.

But, if you've ever gone to the eye doctor and had your pupil dilated, you've no doubt noticed that things look a little fuzzier and light sources tend to have a kind of colorful halo around them. That effect is called chromatic aberration and it turns out cephalopods see the world that way pretty much all the time.

Their pupils tend to have the wide-open shape of a U, W or even a dumbbell in the case of some species of octopi. Such a pupil design doesn't focus the light, but rather, lets it come in from all angles, creating a blurry image. But, according to new research out of the University of California Berkeley (UC Berkeley) and Harvard University, cephalopods can change the depth of their eyeball and move their oddly-shaped pupils around to bring specific wavelengths – i.e. colors – into focus on their retinas.

The oddly shaped pupils of a cuttlefish, squid and octopus from top to bottom(Credit: Roy Caldwell, Klaus Stiefel, Alexander Stubbs)

So even though they don't have the photoreceptor ability to see colors we do, their eyes have adapted to sense color in a completely different way. They may not have clear vision, but they have vision that lets them sense and blend into their surroundings, which may just be a more valuable evolutionary adaptation than sharp sight.

"We propose that these creatures might exploit a ubiquitous source of image degradation in animal eyes, turning a bug into a feature," said UC Berkeley graduate student Alexander Stubbs. "While most organisms evolve ways to minimize this effect, the U-shaped pupils of octopus and their squid and cuttlefish relatives actually maximize this imperfection in their visual system while minimizing other sources of image error, blurring their view of the world but in a color-dependent way and opening the possibility for them to obtain color information."

Stubbs, who's long had an interested in the colorblind/camouflage conundrum of cephalopods worked with his father, Harvard astrophysicist Christopher Stubbs, to develop a computer simulation model that played with different ways in which cephalopod eyes might work. Their findings were published in the journal Proceedings of the National Academy of Sciences online this week.

"We believe we have found an elegant mechanism that could allow these cephalopods to determine the color of their surroundings, despite having a single visual pigment in their retina," Stubbs said. "This is an entirely different scheme than the multi-color visual pigments that are common in humans and many other animals. We hope this study will spur additional behavioral experiments by the cephalopod community."

The UC Berkeley video below shows off the amazing color-changing abilities of cephalopods, with a good close-up of one of the creature's eyes.

The oddly shaped pupils of a cuttlefish, squid and octopus from top to bottom(Credit: Roy Caldwell, Klaus Stiefel, Alexander Stubbs)

Octopi like this might be able to see color by turning an evolutionary "bug" into a benefit(Credit: <a href="https://commons.wikimedia.org/wiki/File:Octopus_at_Kelly_Tarlton%27s.jpg" rel="nofollow">Pseudopanax/Wikimedia Commons</a>)